Enhanced nitrogen removal in SBRs bypassing nitrate generation accomplished by multiple aerobic/anoxic phase pairs.

A lab-scale SBR was used for the study of nitrogen removal from a synthetic wastewater with an ammonium-nitrogen concentration of 50 mg/L. The react phase of the reactor operation was divided into three sets of consecutive aerobic and anoxic periods with a duration ratio of 1:3 (20 min aerobic and 1 h anoxic phase). Under these operating conditions, nitrogen removal was achieved via nitrite i.e. no nitratification (oxidation of nitrite to nitrate) and hence no denitratification (reduction of nitrate to nitrite) was taking place in the aerobic and anoxic phase, respectively. This was attributed to the suppression of the nitrite-oxidizers activity due to the short aerobic phase duration. This presumption was supported by the ever decreasing amount of nitrate-nitrogen generated in the react phase during the transient, even when the activated sludge of the reactor was supplemented with additional nitrite-oxidizers. On the other hand, denitrification was mainly based on stored carbon sources, as long as the organic carbon (provided in the form of acetate) was never accumulated during the anoxic/anaerobic fill phase of the reactor operation.

Long-term effect of total cycle time and aerobic/anoxic phase ratio on nitrogen removal in a sequencing batch reactor.

A laboratory-scale sequencing batch reactor was used to study nitrogen removal from a synthetic wastewater with an ammonium-nitrogen concentration of 50 mg/L. The effect of two key parameters (i.e., the total cycle time and the aerobic/anoxic phase duration ratio) on the performance of the reactor was investigated. Four sets of operational parameters resulting from the combination of 6 and 8 hours of cycle time and 1:1 and 1:3 aerobic/anoxic ratios were studied. Denitrification was based mainly on endogenous carbon sources as long as organic carbon, provided in the form of acetate, was quickly removed from the mixed liquor during the aerobic phase of the reactor operation. In terms of nitrogen removal efficiency, the set of 8 hours of cycle time and 1:3 aerobic/anoxic phase ratio was found to be superior in that it consistently yielded an effluent total nitrogen concentration of less than 15 mg/L (consisting almost exclusively of nitrate-nitrogen) and exhibited a percentage nitrogen removal rate of 77 +/- 2.5%. In terms of wastewater throughput (i.e., wastewater volume treated per day), however, the implementation of 6 hours of cycle time and 1:3 aerobic/anoxic phase ratio was satisfactory for nitrogen removal efficiency (72 +/- 2%), although elevated amounts of ammonium- and nitrite-nitrogen were found in the effluent. It was also demonstrated that under certain operating conditions nitrogen removal via nitrite is observed, as nitratification (i.e., oxidation of nitrite-nitrogen to nitrate) and denitratification (i.e., reduction of nitrate-nitrogen to nitrite) are bypassed.

Comparative study of evaluation of digital arterial circulation by doppler ultrasonic tracing and hand arteriography.

The measurement of DBP and the arteriography of the hand have been used for the estimation of digital arterial circulation and for the disclosure of occlusive processes of the digital and palmar arteries in vascular and, mainly, in arterial disorders of the hands. However, both methods have some limitations; the first might give normal values in the presence of occlusion of digital or palmar arteries while the second is an invasive technic, it cannot be repeated and is followed by a low morbidity. The technic that we are introducing namely the digital arterial flow tracing (DAFT) is a non-invasive method which discloses, with a high accuracy, the occlusive processes of the digital arteries and it could replace to some extent especially in the study of Raynaud's syndrome, hand arteriography. Further experience is required for the refinement of the method and for the study of the palmar arteries.